1. Field of the Invention
The invention relates to a nonvolatile memory capable of high-density and a method of writing data thereto. More particularly, it relates to an EEPROM provided with a single source (a first impurity diffused layer) in common and a single drain (a second impurity diffused layer) in common for plurality of memory cells and a method of writing data thereto.
2. Description of the Related Art
A memory cell comprising a Si substrate provided with a source and drain, an auxiliary gate (AG) formed through the intermediary of an insulating film, a floating gate (FG) formed on the side wall of the AG through the intermediary of an insulating film, and a control gate (CG) provided on the FG and AG through the intermediary of the insulating films has been proposed. In a memory cell array having such memory cells arranged in matrix in the directions of X and Y, each memory cell is provided with a source and a drain.
This type of memory cell has a writing property as shown in FIG. 5.
In FIG. 5, the vertical axis shows a threshold voltage (V.sub.t) after the data are written (after programming) and the horizontal axis shows an auxiliary gate voltage (V.sub.AG). When the data were written, the applied voltage was 5V to the drain and 12V to the control gate for 5 .mu.sec. When the data were erased, the applied voltage was 5V to the drain and 11V to the control gate for 5 msec.
FIG. 6 is an equivalent circuit diagram of the memory cell of this type.